1. Field of the Invention
The invention relates generally to electrostatic chucks for holding a workpiece and, more specifically, to an improved bipolar electrostatic chuck for use in a plasma-enhanced environment and a method for making the improved bipolar electrostatic chuck.
2. Description of the Background Art
Electrostatic chucks are used for holding a workpiece in various applications ranging from holding a sheet of paper in a computer graphics plotter to holding a semiconductor wafer within a semiconductor wafer process chamber. Although electrostatic chucks vary in design, all are based on the principle of applying a voltage to one or more electrodes in the chuck so as to induce opposite polarity charges in the workpiece and electrodes, respectively. The electrostatic attractive force between the opposite charges presses the workpiece against the chuck, thereby retaining the workpiece.
In semiconductor wafer processing equipment, electrostatic chucks are used in process chambers for clamping wafers to a support surface of a pedestal during processing. The pedestal may form both an electrode and a heat sink as used in plasma-enhanced etching, chemical vapor deposition (CVD) or physical vapor deposition (PVD) applications. One form of electrostatic chuck that clamps the wafer to the support surface of the pedestal and establishes optimal heat transfer conditions between the chuck and the wafer is a "bipolar" electrostatic chuck. Bipolar chucks are well known in the art. For example, U.S. Pat. No. 4,384,918 issued May 24, 1983 to Abe discloses a bipolar chuck for use in a plasma-enhanced etch chamber. More specifically, a bipolar chuck contains two, coplanar electrodes embedded beneath the support surface of the pedestal. The two electrodes are biased by either a DC or AC power source. An electric field is created between the two electrodes and is coupled through the wafer. The electric field causes charges to migrate along the underside of the wafer. As such, the wafer and the electrodes accumulate oppositely polarized charges and the wafer is clamped to the support surface of the pedestal. This configuration allows the wafer to be chucked immediately upon placement onto the support surface and without the need for a plasma within the chamber. Therefore, a heat transfer gas is introduced between the wafer and support surface before plasma formation and wafer processing. Additionally, the gas remains at this wafer-to-chuck interface after processing and powering down of the plasma power source affording greater temperature control.
To facilitate plasma excitation, a third electrode is embedded within the pedestal at a location below the electrostatic chuck electrodes. This third electrode (a cathode electrode) is connected to an RF source to produce an electric field within the chamber such that a plasma is excited above the wafer surface. The electric field extends from the cathode electrode to the grounded walls and top (together forming the anode electrode) of the process chamber. The electric field penetrates the chuck electrodes to enter the process chamber. More accurately, the RF energy is capacitively coupled from the cathode electrode to the pair of chuck electrodes and then from the chuck electrodes to the anode.
Typically, a bipolar electrostatic chuck has a gap between the chuck electrodes to prevent a short circuit of the electric field established at the support surface by the chucking voltage. This gap is usually on the order of 0.3 to 5 mm wide. Nonetheless, this rather small gap is sufficiently wide to alter the RF electric field above the support surface of the pedestal. This condition creates a non-uniform plasma above the wafer and a subsequent non-uniformly processed wafer. Consequently, the state of the art does not provide a bipolar electrostatic chuck capable of maintaining uniform plasma conditions.
Therefore, there is a need in the art for an apparatus that is capable of exploiting the advantages of bipolar electrostatic chucks while reducing their inherent characteristic of plasma non-uniformity.